1. Field of the Invention
The present invention relates to a method for forming a capacitor for use in a semiconductor device, and in particular to a method for forming an electrode of the capacitor having a lot of small protrusions on the surface thereof.
2. Description of the Related Art
In recent years, the size of a semiconductor memory is reduced with the high integration of Dynamic Random Access Memories. This causes the area of a memory cell capacitor to be reduced, and consequently a problem arises such that the capacitance of a memory cell is also reduced. Then, in order to achieve sufficient capacitance, there have been proposed several methods of increasing a capacitor area without reducing the thickness of an insulation film.
For example, a method described in Japanese Patent Laid-open Publication No. 4-252018 is shown in FIGS. 1A and 1B. An amorphous insulation film (oxide film) 2 is formed on a substrate 1, and then an amorphous silicon 3 is formed on the insulation film 2. Immediately after formation of the amorphous silicon 3, plasma hydrogen 4 is supplied to the surface of the amorphous silicon 3 without exposing it to the air. The surface atoms of the amorphous silicon 3 are terminated by the hydrogen atoms 5 and become inactive. Therefore, even if it is left in the atmosphere for a long period of time, formation of a natural oxide film would be prevented. Since these hydrogen atoms 5 are dissociated from silicon atoms at a lower temperature than the amorphous silicon layer 3 crystallizes, a clean surface is obtained prior to the crystallization of amorphous silicon. The diffusion speed of the silicon atoms on the clean surface is extremely fast, so that seed grains are formed and grown on the surface of the amorphous silicon 3, and as shown in FIG. 1B there is formed polycrystalline silicon 7 having a lot of hemispherical polycrystalline grains 6 on its surface, resulting in very large surface area. If the polycrystalline silicon 7 formed in this way is used as a storage electrode, the capacitance is approximately twice or more as large as in a case where plasma hydrogen 4 is not supplied to the amorphous silicon 3.
However, the conventional method has practical difficulties since the clean surface of the amorphous silicon 3 is unstable and difficult to hold for a long period of time. Moreover, heat treatment under very high vacuum conditions is needed for grain formation and growth. Therefore, the equipment for forming the polycrystalline silicon 7 is expensive, and temperature control of the heat treatment is not easy because an allowable range of heat treatment temperature is narrow.
In addition, for the conventional method, it is necessary to polycrystallize the amorphous silicon 3 before patterning for formation of the storage electrode because it is very difficult to hold the clean surface of the amorphous silicon 3 under patterning by means of photolithography. This causes the process design to be reduced in flexibility.